1. Technical Field
This disclosure relates generally to catalytic converters, more particularly, to the use of stabilized palladium (+1) as a catalyst.
2. Background Information
Emission standards for unburned contaminants, such as hydrocarbons (that cause adverse health effects), carbon monoxide (which reduces oxygen delivery within the body) and nitrogen oxide (that produces smog), continue to become more stringent. In order to meet such standards, three-way catalysts (TWC) are located in the exhaust gas line of internal combustion engines. Such catalysts promote the oxidation of unburned hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides to nitrogen in the exhaust gas stream.
Ensuring that the materials of the conversion catalysts are available and active to convert exhaust gas throughout the life of the catalyst is an on-going goal. It is most common to find tri-metallic catalyst systems for controlling HC, CO and NOx emissions, but the catalysts cost is high. Additionally, because of the presence of two different group VIB metal compounds in tri-metallic catalyst, the production process is more complicated. Moreover, it is difficult to reclaim the metals from the spent (or used) catalysts, because it is difficult to separate two different group VIB metals with a high yield.
TWC Catalysts typically contain three platinum group metals: platinum, palladium, and rhodium. Platinum and palladium are generally used for CO and hydrocarbon conversion, while rhodium is known for its excellent performance for the reduction of nitric oxide. However, the price of rhodium can be very high and has a tendency to fluctuate with time. Palladium also can reduce NOx; however, it is generally not as effective as rhodium.
Because of its exceptional nitric oxide reduction performance, rhodium is the most common element among the PGMs employed as TWC, but this PGM has the most volatile price.
Therefore, there is a continuing need to provide a TWC that provides sufficient conversion so that stringent HC, NOx, and CO emission limits can be achieved cost-effectively, minimizing the amount of PGM metals required, specially Rh, as the PGM metals are expensive.